Effects of the neonicotinoid insecticides thiamethoxam and imidacloprid on metamorphosis of the toad Rhinella arenarum at environmentally-relevant concentrations

The goal of the present study was to examine the effects of environmentally relevant concentrations of the neonicotinoid insecticides thiamethoxam and imidacloprid on the metamorphosis of the toad Rhinella arenarum. Tadpoles were exposed from stage 27 until completion of metamorphosis to concentrations of thiamethoxam ranging between 1.05 and 1050 mu g/L and concentrations of imidacloprid varying between 3.4 and 3400 mu g/L. The two neonicotinoids were found to act differently at the range of concentrations tested. Thiamethoxam did not markedly alter the final % tadpoles completing metamorphosis but extended by 6-20 days the time needed for tadpoles to complete metamorphosis. The extra number of days required to reach metamorphosis was concentration-dependent between 1.05 and 100.5 mu g/L, and then stable at 20 days between 100.5 and 1005 mu g/L. In contrast, imidacloprid did not significantly interfere with the overall time needed to complete metamorphosis but decreased success of metamorphosis at 3400 mu g/L, the highest concentration tested. Both neonicotinoid concentrations did not markedly alter body size and weight of the newly metamorphosed toads. With a lowest observed effect concentration (LOEC) of 1.05 mu g/L, thiamethoxam may be more likely to impact tadpole development in the wild compared to imidacloprid, which was without any apparent effect at concentrations up to 340 mu g/L (no-observed effect concentration or NOEC). As the influence of thiamethoxam was triggered after tadpoles had reached Stage 39, when metamorphosis is strictly dependent upon thyroid hormones, this observed effect is attributed to result from actions of this neonicotinoid insecticide on the hypothalamic-pituitary-thyroid axis.